LVP Furnace Control

The LVP Furnace Control (found under Other electronics on 13BMD.adl) directly controls the DAC (Digital to Analog Converter) which controls the AC power supply control voltage. This control voltage can be adjusted by either entering a value in the Control Voltage field, or "tweaking" it with the two tweak buttons (see above). A voltage limit can (and should) be set. 

Note: you should always set a volt limit low enough to help reduce the risk of power overshoot.  However, if you forget to raise the limit and find temperature is no longer increasing because of this limit, DO NOT just raise volt limit, as the PID will try to shoot the power up instantaneously based on the accumulated Proportional and Integrated differences.  When you hit the volt limit, lower the temperature first so that you are no longer volt limited, change the limit to a higher value and then increase the temperature.

The actual voltage produced by the power supply is a multiple of the control voltage – 0-10 control volts corresponds to 0-Vmax of the power supply (settings vary). The default ratio is 1:1 (see PID limits, below)

Feedback control is available using the PID (Proportional-Integral-Differential) system. This system can control on either temperature or power. The Setpoint on the PID feedback control window is set to the value you wish to control, whether power or current. The Control PV adjusts the DAC (furnace power supply control voltage).

The Readback PV monitors either the temperature (13BMD:LVP_furnace_calcs.E NPP NMS) or power (13BMD:LVP_furnace_calcs.D NPP NMS) (see furnace_calcs, below).

The limits and some of the calculations are in the PID limits window:

The AC power supply volts and current are in fields A and B, read from Kiethley #2. The resistance is calculated from these values, and the power from the resistance. Field E is the ratio of the Control Voltage to the power supply voltage, and is set here if other than 1:1. The maximum valus for the power supply volts, current, and power are fields F, G, and H, respectively and are also set here. Because the limits are always in terms of control voltage, these fields are converted to volts in fields I, J, and K. Finally, field P tells the PID to use the minimum of I, J, and K, all divided by the control voltage:output voltage ratio, E.

Note: Field E is changed to match the power supply settings, and fields F, G, and H may be user-adjusted.

The PID update window is used to transport control parameters. 

Note that the upper left button (set as “Passive”) must be changed to a certain update frequency (fro example 1 second) in order for the parameters to be transported.  When the state is set as “Passive”, PID control will NOT be in control!!

 

 The PID parameters (PID feedback parameters) need to be adjusted when either changing the type of furnace, or when changing from temperature to power control. Note that KD is always zero or negative; the other terms are positive.

Controlled by

KP

KI

KD

Temperature

0.001 – 0.005

0.5 – 1.0

0

Power

.04

30

-15

 The plot window allows the user to monitor how sell the controlled parameter follows the setpoint. You must manually adjust the range for both the setpoint (Control PV) and the controlled value (Readback PV). These will be in real units (°C or Watts). When the ranges are changed, press replot to reset the scales.

Note: Current MEDM has a bug: after you input new display maximum and/or display minimum, the plot will not re-scale when the “Replot” button is pressed.   There are two ways to get around it.  Either re-size the plot window by dragging it from one corner, or kill the current plot window and re-open it.

The furnace calcs does just what its title says. Notice that field D is the power, and E is the temperature. T is read directly from Keithley #1 channel 10 (DMM1Ch10_calc.) If a different type of thermocouple is used, this needs to be changed here to the proper channel.

 

To summarize the limit situation:

Ultimately, the only thing that is controlled is the power supply control voltage, which is the output of the DAC.

The DAC is the minimum of [(LVP furnace control control voltage limit) and (PID update.P)]

PID update.P is the minimum of [(desired output) and (PID limits.P)]

PID limits.P is the minimum of [(max volts), (max amps´ resistance) and sqrt(max power´ resistance)], all converted to control volt units by dividing by E.

So actually, this is the minimum of [(LVP furnace control control voltage limit), (max volts)/E, (max amps´ resistance)/E, and sqrt(max power´ resistance)/E].

The items in bold are all user controlled. If the power or temperature stops increasing, all of these parameters must be checked and altered if necessary.